Packed images used as background don't show up when reopening the file
Do you have Load UI unchecked in the User Preferences -> File tab?  That is the only way I can reproduce this. 



Re-checking Load UI, will make blender open a file using the layout of when the file was last saved, including background images

From the Blender Manual:


  Load UI
  
  Inside each .blend file, Blender saves the user interface arrangement. By default, this saved UI is loaded, overriding any user defaults or current screen layouts that you have. If you want to work on the blend file using your own defaults, start a fresh Blender, then open the file browser and turn off the Load UI button, and then open the file.